1. Field of the Invention
The invention relates to rod-type polysilicon having improved breaking properties, obtained according to the Siemens deposition method.
2. Background Art
Polycrystalline silicon (polysilicon) serves as a starting material for producing monocrystalline silicon for semiconductors according to the Czochralski or floating zone method, and also for producing mono- or multicrystalline silicon according to various pulling and casting methods for the production of solar cells for photovoltaics.
In this case, the polysilicon is principally produced by deposition using trichlorosilane according to the so-called Siemens method. In this method, in a bell-shaped reactor, the so-called “Siemens reactor”, thin filament rods composed of silicon are heated by direct passage of current and brought into contact with a reaction gas composed of hydrogen and one or more silicon-containing components. In this case, a preferred raw material is trichlorosilane (SiHCl3) or a mixture thereof with dichlorosilane (SiH2Cl2).
The filament rods are inserted perpendicularly in electrodes which are situated on the base of the reactor and via which the connection to the power supply is effected. High-purity polysilicon deposits on the heated filament rods and on the horizontal bridge that respectively connects two rods, as a result of which the rod diameter increases with time.
After the desired diameter has been attained, the further addition of Si-containing components ceases. The reactor is subsequently purged in order to remove all gaseous reaction products and the residues of the Si-containing components. The purging is generally effected either using hydrogen or using an inert gas such as nitrogen or argon. After purging, the inflow of the purging gas is stopped and the supply of energy is reduced, either abruptly or with a specific ramp to zero, and the resulting Si rods cool then to the ambient temperature.
After the rods have cooled and if the purging was not effected using inert gas, hydrogen in the reactor bell is replaced by an inert gas, the deposition reactor is opened and the carrier bodies with the polysilicon rods are removed from the reactor.
For various uses of the polysilicon rods it is then necessary to break the rods into small pieces in a subsequent step. Si rods produced according to the conventional Siemens method are very hard and therefore require high forces to break them apart. This has the effect that, during the corresponding breaking method, Si fragments on the surface are significantly contaminated with material of breaking tools. Moreover, after a breaking, the intention is for as many Si fragments as possible to be in a preselected size range. The process of breaking Si rods according to the prior art gives rise to many fragments which do not correspond to the desired size and, consequently, have to be sold at significantly reduced prices.
EP 0329163 describes a method for improving the breaking properties of polysilicon wherein the Si rods, after deposition, are once again heated and quenched or subjected to a compression wave in an aftertreatment. However, this method is associated with very high technical complexity, high costs, and risk of contamination.
During the production of thick polycrystalline Si rods it can relatively often be observed that they tilt from the mounts and fall over during the cooling phase in the reactor after the deposition. This phenomenon delays the production process considerably, since the rods that have fallen over can only be removed from the reactor with additional complexity. Furthermore, this also gives rise to high financial damage since tilted or collapsed Si rods can no longer be processed further as envisaged. The financial damage is particularly high in the production of polysilicon for the solar industry, because this material is normally used without additional cleaning steps. Rods that have fallen over subsequently have to be subjected to complex cleaning, which makes the solar material significantly more expensive.
U.S. Pat. No. 5,593,465 discloses arranging at least one spring element between the current feed and the electrode holder, the spring element permitting a movement of the electrode holder relative to the current feed and cushioning this movement. This arrangement is intended to minimize tilting and falling of the Si rods. What is disadvantageous about this solution, however, is the high technical complexity and the considerable costs associated therewith.
U.S. Pat. No. 6,639,192 recommends the use of carbon electrodes having a high thermal conductivity in order to prevent the rods from falling over in the reactor. This solution has the serious disadvantage, however, that, owing to the high thermal conductivity of carbon electrodes, the latter are not overgrown or are overgrown only little by the silicon during the deposition process. This has the effect that the rods can easily fall over as early as at the beginning of the deposition.